A conventional forced cold air circulating system has circulated cold air by sending with a fan the air cooled by a cooler such as a cooling coil forcibly from a blowing port into a cooling chamber in which an object to be cooled is placed, withdrawing the cold air whose temperature has risen due to heat exchange with the object to be cooled from a suction port to the cooler, cooling the air with the cooler again and sending the air to the cooling chamber with the fan. In this system, the cold air is blown against the surface of the object to be cooled, thereby cooling the object while removing moisture as well as hot air from the object.
Accordingly, the forced cold air circulating system has the following problems. 1) As the object to be cooled dries, its natural moisture is taken away. In the case where the object to be cooled is a food material, its taste and quality deteriorate. 2) The moisture is taken from the object to be cooled, so that, in a freezing temperature range, ice crystals attract each other and grow into larger crystals, thus swelling and also engulfing intracellular elements of the object to be cooled, resulting in degeneration of the object. 3) Since the circulating path of the cold air is fixed, the time during which the air is in contact with the object to be cooled is short, making it difficult to conduct quick cooling. 4) Because of the high speed of cold air, powder of some objects to be cooled may be scattered and make an interior dirty. 5) The moisture taken from the object to be cooled returns to the cooler, causing a frost deposition. This necessitates defrosting. 6) Since the interior temperature rises during defrosting, fine ice crystals start melting. The melted ice crystals freeze to form large crystals, which destroy the cells, thus changing the object to be cooled. When the object is preserved for a long time, its elements become broken.
In order to solve these problems, JP 2852300 B (Patent document 1) and JP 3366977 B (Patent document 2) have suggested cooling devices that do not circulate cold air forcibly. In these cooling devices, a cooler is provided on a side of one wall in a chamber sealed by a heat-insulating housing, a front surface of the cooler is provided with a cooling fan, a space in front of the cooling fan serves as a cooling chamber, and cooled air present near the cooler is withdrawn from a back surface of the cooling fan and allowed to flow into the cooling chamber. The cooled air in the cooling chamber is not circulated forcibly to the cooler, and a heat exchange by collision of molecules between the cooling chamber and a cooling portion including the cooler is carried out at an interface between air layers of the cooling portion and the cooling chamber. Thus, the cooling chamber has a saturated water vapor pressure and is not dry, so that a slight amount of moisture on the surface of the object to be cooled is frozen instantaneously to form a thin ice barrier over the entire surface. This makes it possible to keep the ice crystals in the object to be cooled microscopically, thereby avoiding the degeneration of the object.
According to the description in JP 3366977 B, it is appropriate that a gap between a back surface of the cooling coil serving as the cooler and the wall surface of the chamber range from 20 to 50 mm. A gap smaller than the above does not allow a sufficient amount of cold air to be withdrawn, whereas an excessively large gap causes the cold air to be distributed in that gap, preventing the guidance of the cold air to the space behind the fan.
However, the studies conducted by the inventors of the present invention have revealed not only that the gap with the above-noted numerical range does not produce a sufficient cooling effect but also that there is a condition that should be satisfied in order to provide a practical cooling device. In other words, there is a problem that it is impossible or insufficient for achieving a cooling device at a practical level to satisfy only the condition described in the conventional documents mentioned above.
Patent document 1: JP 2852300 B
Patent document 2: JP 3366977 B